Substantially pure cyclic phosphonitrilic chloride trimer is a very desirable starting material, for example, in the polymerization reaction to form linear phosphonitrilic chloride polymers. Use of such high purity trimer enables formation of high molecular weight linear phosphonitrilic chloride polymers without the problems of crosslinking and gel formation associated with polymerization of mixtures of the trimer with various other phosphonitrilic chlorides such as the tetramer, pentamer and the like. Unfortunately, reactions known in the art for making the cyclic trimer result in mixtures of the trimer, tetramer, and other higher cyclics as well as some linear phosphazenes.
The general approach of prior art methods of separation of isomers of chlorophosphazene has involved utilization of the differences of boiling points of the isomers or exploitation of the different reactivities of the isomers with aqueous bases. Specific purification methods have involved extracting chlorophosphazene from petroleum ether solutions with sulfuric acid, U.S. Pat. No. 3,008,799; separating trimer and tetramer from the produced mixtures through distillation using a spinning band column, U.S. Pat. No. 3,379,510; contacting molten chlorophosphazenes with inert solvent vapor so as to selectively vaporize and entrain cyclic trimer and then condensing the vapor to form a solution of trimer in the solvent from which trimer together with some tetramer can be recovered, U.S. Pat. No. 3,677,720; and steam distilling chlorophosphazenes resulting in hydrolysis of the tetramer and higher forms and hence separation of the trimer, Radosavljevic, et al, Glas. Hem. Drus., Beograd 1971, 36(5-6), 189-91, Chemical Abstracts, Vol. 77, p. 540, 159648d (1972). In addition, U.S. Pat. No. 3,378,353 describes a controlled crystallization procedure wherein trimeric phosphonitrilic chloride is separated from mixtures thereof with tetramer in proportions of trimer to tetramer of at least 70:30 by weight by dissolving the mixture in an organic solvent such as petroleum ether, gasoline or kerosene, monochlorobenzene, carbon tetrachloride, symmetrical tetrachloroethane, dioxane, benzene, toluene, xylene and carbon disulfide, at a temperature sufficient to effect complete dissolution (generally about 50.degree. to about 70.degree. C.), cooling the solution until the constant temperature is reached at which the trimer begins to crystallize out (generally between about 20.degree. and about 50.degree. C.), and filtering the solution substantially at the constant temperature to remove the crystals of substantially pure trimer which separate at that temperature; and U.S. Pat. No. 2,862,799 describes a process for separating substantially pure tri-(phosphonitrilic chloride) from a mixture comprising this compound and at least one of its higher polymers by contacting the mixture with a liquid monohydric alcohol at a temperature and for a period of time sufficient to cause said higher polymer to pass into the liquid phase of the alcohol while leaving substantially pure tri-(phosphonitrilic chloride) in the solid state, and separating said tri-(phosphonitrilic chloride) from the liquid phase.